1. Field of the Invention
The present invention relates to a combustion state detecting apparatus for detecting the combustion state of an internal-combustion engine by detecting the changes in the quantity of ions observed during the combustion in the internal-combustion engine and, more particularly, to a combustion state detecting apparatus for an internal-combustion engine which is capable of preventing pre-ignition or a drop in bias voltage at the time of energizing an ignition coil so as to obviate control errors and to ensure sound bias voltage for detecting ion current especially in an internal-combustion engine of low voltage distribution.
2. Description of Related Art
Generally, in an internal-combustion engine driven by a plurality of cylinders, a fuel-air mixture composed of fuel and air which has been introduced into the combustion chamber of each cylinder is compressed as a piston moves up, and high voltage for ignition is applied to a spark plug installed in the combustion chamber to generate an electric spark so as to burn the fuel-air mixture; the explosive force produced when the fuel-air mixture is burnt is converted to the force which pushes the piston down is taken out as a rotary output of the internal-combustion engine.
It is known that, when the combustion takes place in the combustion chamber, the molecules in the combustion chamber are ionized, and therefore, applying bias voltage to ionic current detecting electrodes, which are usually spark plug electrodes and which are installed in the combustion chamber, causes ions with electric charges to move in the form of ionic current between spark plug electrodes.
It is also known that the ionic current sensitively reacts to the combustion state in the combustion chamber, making it possible to detect a combustion state in the internal-combustion engine by detecting the state in which the ionic current is generated.
This type of combustion state detecting apparatus for an internal-combustion engine is described in, for example, Japanese Unexamined Patent Publication No. 4-191465 wherein a spark plug is employed as an electrode for detecting ionic current, and a combustion failure including a misfire is detected from the quantity of ionic current detected immediately after ignition.
FIG. 8 is a circuit configuration diagram illustrative of an example of a conventional combustion state detecting apparatus for an internal-combustion engine based on low voltage distribution.
In FIG. 8, the cathode of an in-car battery 1 is connected to one end of a primary winding 2a of an ignition coil 2, the other end of the primary winding 2a being connected to the ground via an emitter-grounded power transistor 3 for cutting off the supply of primary current.
A secondary winding 2b of the ignition coil 2 constitutes, together with the primary winding 2a, a transformer; the high voltage end of the secondary winding 2b is connected to one end of a spark plug 4 corresponding to each cylinder, not shown, to output high voltage of negative polarity at the time of ignition control.
The spark plug 4 composed of opposed electrodes discharges to ignite the fuel-air mixture in a cylinder when the high voltage for ignition is applied thereto.
In this drawing, only a pair of the ignition coil 2 and the spark plug 4 are shown as a representative of those ignition coils 2 and spark plugs 4 which are provided for respective cylinders.
The low voltage end of the secondary winding 2b is connected to an ionic current detecting circuit 10. The ionic current detecting circuit 10 applies a bias voltage of positive polarity, which is the opposite polarity from the ignition polarity, to the spark plug 4 via the secondary winding 2b and it detects the ionic current which corresponds to the quantity of ions generated at the time of combustion.
The ionic current detecting circuit 10 includes: a biasing means, namely, a capacitor C connected to the low voltage end of the secondary winding 2b; a diode D inserted between the capacitor C and the ground; a resistor R connected in parallel to the diode D; and a zener diode DZ for limiting voltage which is connected in parallel to the capacitor C and the diode D.
The series circuit composed of the capacitor C and the diode D and the zener diode DZ connected in parallel to the series circuit are inserted between the low voltage end of the secondary winding 2b and the ground to constitute a charging path for charging the capacitor C with the bias voltage at the time when ignition current is produced.
The capacitor C is charged with the secondary current which flows via the spark plug 4 discharged under the high voltage output from the secondary winding 2b when the power transistor 3 is turned OFF, i.e. when the current supplied to the primary winding 2a is cut off. The charging voltage is limited to a predetermined bias voltage, e.g. a few hundred volts, by the zener diode DZ; it functions as the biasing means, i.e. the power supply, for detecting ionic current.
The resistor R in the ionic current detecting circuit 10 converts the ionic current provided by the bias voltage to a voltage which is supplied as an ionic current detection signal Ei to an electronic control unit (ECU) 20.
The ECU 20 comprised of a microprocessor determines the combustion state of the internal-combustion engine according to the ionic current detection signal Ei; if it detects a bad combustion state, then it carries out appropriate corrective measures to prevent a problem.
The ECU 20 also computes the ignition timing, etc. according to the operating conditions obtained through various sensors, not shown, and issues an ignition signal P for the power transistor 3, fuel injection signals to the injectors, not shown, of the respective cylinders, and driving signals to various actuators such as a throttle valve and an ISC valve.
FIG. 9 through FIG. 11 are explanatory drawings illustrative of the path along which current flows into the secondary winding 2b and the ionic current detecting circuit 10; FIG. 9 and FIG. 10 illustrate the path, which is indicated by the solid line, of secondary current I2 flowing under the high voltage at the time when the spark plug 4 discharges, that is, during the ignition control; and FIG. 11 illustrates the path, which is indicated by the dashed line, of ionic current i running under the bias voltage at the time when the ionic current is detected.
Referring now to FIG. 9 through FIG. 11, the operation of the conventional combustion state detecting apparatus for an internal-combustion engine shown in FIG. 8 will be described.
Normally, the ECU 20 computes the ignition timing, etc. according to operating conditions and applies the ignition signal P to the base of the power transistor 3 at a target control timing so as to turn the power transistor 3 ON/OFF.
Thus, the power transistor 3 cuts off the supply of the primary current flowing into the primary winding 2a of the ignition coil 2 in order to boost the primary voltage and to generate the high voltage, e.g. a few tens of kilovolts, for ignition at the high voltage end of the secondary winding 2b.
This secondary voltage is applied to the spark plug 4 in each cylinder to generate a discharge spark in the combustion chamber of the cylinder under ignition control, thereby burning the fuel-air mixture. At this time, if the combustion state is normal, then a predetermined quantity of ions are produced around the spark plug and in the combustion chamber.
The secondary current I2 triggered by the discharge of the spark plug 4 at the time of ignition flows along the path indicated by the solid line shown in FIG. 9 and charges the capacitor C, which provides the bias power supply, via the charging path in the ionic current detecting circuit 10.
Then, as soon as the bias voltage of the capacitor C exceeds the zener voltage of the zener diode DZ, the secondary current I2 flows along the path indicated by the solid line in FIG. 10, and the bias voltage of the capacitor C is limited by the zener voltage of the zener diode DZ. The bias voltage of the capacitor C is set to an arbitrary predetermined value by the circuit characteristic of the zener diode DZ.
The bias voltage thus charged in the capacitor C is applied to the spark plug 4 of a cylinder which has just been subjected to the ignition control, i.e. combustion, via the secondary winding 2b, causing the ionic current i, which corresponds to the quantity of ions produced at the time of combustion, flows as indicated by the dashed line in FIG. 11. At this time, the ions move between the electrodes of the spark plug 4, and the capacitor C discharges.
The ionic current i is detected as the ionic current detection signal Ei by the voltage drop across the resistor R. The ECU 20 determines the combustion state of each cylinder according to the ionic current detection signal Ei and computes appropriate control parameters such as ignition timings in accordance with the operating conditions and the combustion states as previously described.
When, however, the supply of current to the primary winding 2a of the ignition coil 2 is begun, the high voltage end of the secondary winding 2b develops a positive voltage, which is opposite from that at the time of ignition.
This voltage has the same polarity as the bias voltage; therefore, if the bias voltage is superimposed, then there is a danger that discharge may take place between the opposed electrodes of the spark plug 4.
If the discharge takes place at the spark plug 4 at the time of the start of energizing the ignition coil 2, then a control error due to pre-ignition occurs and also, the capacitor C in the ionic current detecting circuit 10 wastefully discharges.
Furthermore, the discharge of the capacitor C causes a drop in the bias voltage, resulting in deteriorated sensitivity for detecting the ionic current i, and the current at the time of the discharge is erroneously detected as the ionic current i.
Thus, the conventional combustion state detecting apparatus for an internal-combustion engine has been posing a problem in that control errors attributable to pre-ignition, deteriorated sensitivity for detecting ionic current, and detection errors cannot be prevented because no measures have been made against the discharge of the bias voltage which may occur at the start of energizing the ignition coil 2.